The invention relates to an improvement to a method for the preparation of cyclohexanol and/or cyclohexanone by the steps of hydrogenating benzene to cyclohexene, hydrating cyclohexene thus obtained to cyclohexanol and/or oxidating it to cyclohexanone, recovering product cyclohexanol and/or cyclohexanone, and returning unconverted benzene and/or cyclohexene to the hydrogenation step.
Such a method is known from Japanese Patent application No. 53,090,242, which application has been laid open to inspection (Derwent Abstract No 65968A), for the preparation of cyclohexanone. However, a serious problem is encountered in practicing this known method in that by-product cyclohexane is also formed in the hydrogenation of benzene to cyclohexene, unless a low degree of benzene conversion is applied. Such a low degree of benzene conversion is generally not economically justifiable.
Therefore, in the known method, cyclohexane must be separated from the benzene-cyclohexene-cyclohexane containing mixture remaining after recovery of cyclohexanol from the hydration reaction mixture and/or of cyclohexanone from the oxidation reaction mixture prior to recycle of the remaining benzene-cyclohexene containing mixture to the hydrogenation step. However, cyclohexane is very difficult to separate from mixtures containing benzene and/or cyclohexene, and requires relatively expensive separation techniques such as extractive distillation. This is due to the azeotrope formation between benzene, cyclohexene, cyclohexane and/or water.
Faced with this choice, it is generally preferable when practicing the known method to maintain the degree of benzene conversion relatively low, for instance lower dan 10% in order to permit only a minimum quantity of cyclohexane to be formed. However, whether such a low degree of conversion is utilized, or benzene separation is effected by expensive methods such as extractive distillation, an economic process for the preparation of cyclohexanol and/or cyclohexanone from benzene has not been achieved.